The present invention relates to an oscillation circuit for generating two output signals having a phase difference of 90°.
Normally, a receiver converts a received signal of a radio frequency to a predetermined intermediate frequency fIF by using a local signal generated by a local oscillator. For example, when a broadcast station A of a frequency fA is received, the local oscillator generates a local signal of a frequency fLO satisfying the relationship that (fA−fLO) is equal to fIF. Here, the local signal converts, to an intermediate frequency fIF, a received signal of a frequency fB satisfying the relationship that (fLO−fB) is equal to fIF. Therefore, when a signal of another broadcast station B is located at the position corresponding to the frequency fB, a phenomenon occurs where the broadcast station B is also received while superposed on the broadcast station A. The reception of the broadcast station B as described above is called image reception.
Furthermore, there also exists a direct conversion system of mixing a received signal of a radio frequency with a local signal having the same frequency as the received signal concerned and directly converting the signals to a low frequency signal corresponding to the human auditory range. According to this system, the intermediate frequency is not required, and thus the circuit construction is simple.
In order to construct an image rejection mixer for removing the image described above, local signals having a phase difference of π/2 [rad], that is, 90° therebetween are required. Furthermore, when the direct conversion is carried out by using a mixer for demodulating an I/Q composite signal, local signals having a phase difference of 90° are required.
According to a method adopted by a normal receiver, an LC oscillation signal is subjected to halved frequency division to obtain signals having a phase difference of 90°. FIG. 1 is a timing chart showing this method. On the basis of a signal SG having the frequency corresponding to the LC oscillator signal, a halved frequency division circuit generates a signal SG2 whose state is inverted at the rising edge of the signal SG and also a signal SG2′ whose state is inverted at the falling edge of the signal SG. As described above, the signals SG2 and SG2′ having a phase difference of 90° are obtained.
According to this method, the frequency which is twice as high as a desired frequency is generated in the LC oscillator. Here, when the oscillation frequency of the oscillator is set to the same as the local signal, the frequency of the local signal fluctuates with an input signal having a high intensity, so that it is difficult to carry out direct conversion. The direct conversion is favorable as the method of generating the double frequency described above because this problem does not occur.
According to another conventional method of achieving signals having a phase difference of 90°, phase shifting is carried out for the LC oscillator signal by using RC filters. In this method, a high pass filter (HPF) and a low pass filter (LPF) are constructed by RC filters. In the case of the HPF, the phase of an output signal advances from the input signal of the cut-off frequency by 45°. Conversely, in the case of the LPF, the phase of the output signal is delayed from the input signal by 45°. Therefore, the LC oscillator signal is input to the HPF and LPF in which the cut-off frequency is set to the frequency of the LC oscillator signal, whereby two signals having a phase difference of 90° are obtained as the output signals of both the filters.
The construction of subjecting the LC oscillator signal described above to halved frequency division is suitable for direct conversion. In addition, the oscillation frequency can be varied by constructing a capacitor constituting the LC oscillator with a variable condenser or the like, and thus signals having a phase difference of 90° can be obtained across a broad band. However, the LC oscillator has a problem that it is not suitably constructed as a semiconductor integrated circuit. Particularly, in order to carry out reception based on broadband such as television or the like, plural LC oscillators are required, and it is difficult to install them into an LSI chip.
Furthermore, the construction using the RC filter as described above has a disadvantage that the frequencies of the signals having a phase difference of 90° are restricted by the cut-off frequency of the RC filter and thus it is limited to a narrow band. Furthermore, the oscillation frequency of the LC oscillator is coincident with the reception frequency in the above-described construction, and thus there is a problem that this construction is not suitable for the direct conversion. Furthermore, there is also a problem that this construction is not suitable for integration.